1. Field of the Invention
The present invention generally relates to a monochrome camera and more particularly to an apparatus for providing highlight compensation for monochrome cameras such that white-wash is minimized in an image formed in an image display region.
2. Description of the Related Art
Conventional monochrome (e.g., black-and-white (B/W)) cameras typically include a B/W chip set. However, such a chip set does not typically include any backlight compensation circuitry. This presents a problem when the camera views a backlight scene with images in a dimly lit foreground since such images do not appear on the output video with sharp, or clear details.
Another problem is that, even in a color camera having an automatic gain circuit (AGC) having a backlight compensation circuit therein, normal backlight compensation averages more heavily a window region in a video screen. This type of averaging causes increased video gain even if a strong light source is present outside the window. Specifically, in color cameras, backlight compensation is a feature of a conventional AGC and as mentioned above is found therein. A window (WND) pulse from a sync generator, defines a predetermined (e..g, square center) region of interest.
When this WND pulse is fed to the AGC, the AGC weighs the center region more heavily than the surrounding region. Thus, the AGC averages the window region to determine its operating point rather than the entire field-of-view. However, as the gain increases across the video screen, the light source white-washes the video image around its region. This is a problem if the viewer is observing important images around the light source. Thus, viewing is made difficult.
Thus, conventional monochrome cameras use a single chip processor to produce a monochrome video signal and hitherto the invention, such processors lacked the backlight compensation circuitry even found in a color camera.
Further, prior to the invention, the color camera""s AGC suffered from the above-mentioned problem of white-wash.
It is therefore an object of the present invention to provide a monochrome camera, such as a black-and-white (B/W) camera, with a capability of producing B/W backlight compensation.
Another object is to provide a highlight compensation circuit in which the above-mentioned problem of white-wash is overcome.
In a first aspect of the present invention, a monochrome camera is provided with a highlight compensation mechanism in which an external circuit has been added to a conventional single-chip processor, to develop backlight compensation. Unlike the backlight compensation feature found in conventional color camera chip sets, in the invention highlight compensation is external to an automatic gain circuit (AGC) averaging loop. The advantage of having this circuit outside the AGC is that it prevents white-wash in the video image.
Specifically, as mentioned above, normal backlight compensation averages more heavily a window region in the video screen. This type of averaging causes the video gain to increase even if a strong light source is present outside the window. As the gain increases across the video screen, the light source washes white the video image in its region. This does not occur in the present invention including the highlight compensation mechanism since such a mechanism only increases the gain within the window region. Thus, with the present invention, a monochrome camera is provided which reliably produces monochrome backlight compensation.
Further, there is no white-wash encountered in the image formed in the region of the display. Specifically, the white-wash problem encountered with backlight compensation may be eliminated by using the highlight compensation feature of the present invention.
Another advantage of the highlight compensation feature is that it can be applied to any region within the video field without compromising the averaging process of the AGC. The highlight compensation region may be advantageously made to have any size, and the magnitude of the gain within the region may be advantageously set by the highlight compensation circuit.